Constant velocity joints are typically employed in automotive axial drive shafts, and especially in front-wheel-drive vehicles between the transaxle differential and the driving wheel. The CV joints transmit a torque at various speeds, angles and telescopic positions between a first shaft member and a second shaft member.
Current development in the field of CV joints is primarily directed toward improving performance of the CV joint by increasing the torque transmitting capability of the CV joint and reducing noise and vibration of the CV joint. The improved performance of the CV joint has resulted in an increased cost to manufacture the CV joint. However, there is still a demand for inexpensive and easily produced CV joints.
There are many types of CV joints. One CV joint configuration includes a rotatable first shaft that has an axle portion and a pocket end disposed at an end of the axle portion. The first shaft is rotatable about a first longitudinal axis. The pocket end is disposed within a joint housing that is disposed on an end of a second shaft. The second shaft is rotatable about a second longitudinal axis. The joint housing includes a plurality of grooves disposed within a central bore and extending along the second longitudinal axis. The pocket end includes a diameter transverse to the first longitudinal axis that is greater than a diameter of the axle portion of the first shaft. The pocket end defines a plurality of semi-spherical pockets that are circumferentially and radially disposed about the first longitudinal axis in a configuration that is complementary relative to the plurality of grooves. As such, the number of pockets is equal to the number of grooves. The CV joint includes a plurality of spherical drive balls with one drive ball rotatably disposed within each pocket and corresponding groove. The pocket end and first shaft are moveable and may be articulated or stroked axially relative to the second shaft. As the CV joint is articulated (i.e., as the second shaft is moved relative to the first shaft by axial stroking or angulation) the drive balls roll back and forth along the grooves and rotate within the pockets.
The drive balls include a spherical shape. The pockets include a non-spherical shape, and may be defined by a gothic arch rotated about a central axis of each of the drive balls. The central axis of each of the drive balls passes through a center of the drive balls and the second longitudinal axis, and is disposed perpendicularly to the second longitudinal axis. Each of the drive balls contacts the pockets along an annular contact interface, i.e., a circular linear ring extending around the pocket.
The CV joint includes a conformity ratio, C, defined as the radius of the pocket, rp, divided by the radius of the drive ball, rb, such that C=rp/rb. It should be appreciated that if the radius of the pocket is equal to the radius of the drive ball, the conformity ratio is equal to 1. The conformity ratio may be described in terms of two principle conformity ratios, which are measured in mutually perpendicular planes including principle radii of curvature of the pocket. The first principle conformity ratio or transverse conformity ratio is measured along a plane perpendicular to the first longitudinal axis. The second principle conformity ratio or axial conformity ratio is measured along a plane parallel to the first longitudinal axis and perpendicular to the central axis of the ball and pocket. These ratios are measured at the point of contact of the ball and pocket.
Typically, the conformity ratio for CV joints having spherical pockets is between 1.01 and 1.04, including both the axial conformity ratio and the transverse conformity ratio. In the style of CV joints described above, i.e., ball and pocket style CV joints, both principle conformity ratios between the pockets and the drive balls being relatively close produces a higher amount of friction than in the other styles of CV joints. Higher friction within the CV joint reduces the overall performance of the CV joint, and is therefore undesirable.
Thus, it is desirable to provide ball and pocket style CV joints having reduced friction and improved joint performance.